As promised by Ollopa UMSF folks were allowed to attend. As it transpired I think I could have stayed for the entire day but I had to get back to work for the afternoon so I missed out on the sessions after lunch. These are just my notes so they come with absolutely no warranty whatsoever. Todays Agenda:

John Mather  introductionGave a quick introduction and commented that he is glad to see the telescope that originally was scetched out on a blackboard in 1996 finally getting built with a remarkably similar set of features to the original rough design. He cracked a joke about the fact that the original budget was laughed at by some people who have since been proven correct but emphasized that in adjusted terms the project is now 2x the original budget  so it is over the original estimate but it is now realistic and being held tightly.

He made some brief comments about the relevance to JWST's mission of Spitzers recent work, the Hubble Ultra Deep Field and transit Exoplanets. He also made mention of the recent (probable) pair-instability super nova (SN 2006 GY) (he called it a Super Duper Nova ) that has shown that this type of nova may be much more common than had been thought. Their immense brightness and this indication of their frequency pushes back JWST's reach even further out the edge.

You could tell that he was very pleased with the project. He then handed over briefly to Gillian Wright (I think) the co-Lead of the MIRI Science Team who briefly thanked the combined team for enabling a full day free of ITAR restrictions so everyone could attend. I was surprised that this got to the top of the agenda but I'm not complaining  I got a free pass in partly because of folks figuring this out. At that point she handed over to John Gardiner  GSFC Senior Project Scientist for the JWST

JWST Science Objectives - GardinerSTScI will be managing the operations for JWST. (and as an aside go to http://www.stsci.edu/jwst/ for outline data)

JG gave a general purpose JWST overview for the masses  not much new in this but it was a nice intro for those outsiders (like me) who were allowed in for the day.

I didn't pick up much new (ie that isn't on the STScI web site) for the first three mission objectives:

The End of the Dark Ages: First Light and Reionization

The assembly of Galaxies

The Birth of Stars and Protoplanetary Systems

But he had quite an interesting segment on the final item.

Planetary Systems and the Origins of Life

In particular he specifically indicated that JWST will be targeting ExoMoons aswell as ExoPlanets for detection and spectroscopic analysis via combinations of inferior and anterior transits. He mentioned the recent Spitzer 8micron planetary surface mapping of HD 189733b and indicated that JWST should be able to do something similar for Earth like planets in the habitable zone of comparable stars.

He mentioned that coronograph assisted imaging should significantly expand the percentage of systems where JWST will be able to go planet hunting but did not go into much detail. (Later on in the morning Peter Jakobsen pointed out that the lack of a coronograph on NIRSpec means that the there are some additional constraints on what can be done in terms of analysis of non transit exoplanets)

Mission overview, Observatory Overview and Reformulation Summary - Menzel & GiampaoliThis was mega detailed and I probably missed more than 50% because I simply couldn't keep up while making notes. Fantastic stuff though from my point of view and I'd go back to one of Mike's talks in a heartbeat, he reminds me a bit of Rob Manning  seriously passionate about his machines. Mike said that the data was slightly edited to comply with ITAR because all of us foreigners where there but honestly I wouldn't have noticed if he hadn't specifically pointed out the one spot where some hard numbers had been removed. Anyway some details.

All 10 enabling technologies for the mission have now completed "Technology non-advocate review" and are ready for TRL6 more than 12 months ahead of schedule.

The major news however is the re-formulation of the project to mitigate some cost and program risks. Mike gave an incredibly detailed explanation of the changes that have been made, whay the were needed and what the impact would be. These were mostly in the arena of decoupling various test processes, equipment and resources (such as test facilities) so that schedule dependencies and interlocks could be removed. The argument and description of the final result was compelling and the major concern that I had when first hearing of this reformulation (that testing was being reduced or that the team was accepting lower specifications) certainly seem to me to not be true now that I have seen this new plan. In particular these changes have enabled the addition of more vibration analysis, cryocycling and cryometrology testing because programmatic interlocks and resource scheduling are now much simpler.

Some interesting news on the spacecraft design front are the physical changes that have been introduced with the B2 structural changes. For reference B1 is basically the layout that the large scale model is built to. In particular the following significant (ie visibly noticeable) items have been modified:

The Sunshield has changed from a 3-2-2 layout to a 4-2 layout.

Sunshield deployment mechanics now follows a 10 folding stage process using what NG call a unitized pallet system (this appears to me to be a rigid shell like shield enclosure for launch and final structure rigidity). This is down from the 24 folding stages in the video we've all seen so this seems to represent a major simplification in the deployment mechanics.

The deployed cryo-cooler\radiator system from B1 is now a fixed unit, again reducing the deployment mechanical complexity.

The Solar panels have been moved to a single tail-dragger format from the dual side wing format on the B1 layout.

The Bib (rhomboidal stray light shield at the base of the main mirror) and the Frill (stray light shield around the top rim of the main mirror) have been made larger.

Lots of mass savings have been made  in total about 100kg gross has been removed but some of that has been clawed back elsewhere.

Tuned Magnetic Dampers have been added to the secondary mirror mount  these were needed to closeout on a couple of problematic harmonics from the reaction wheels that were preventing the team meeting 70% engineering margins. (I think)

Overall project performance metrics are green or will be following the B2 structural changes.

Mass is now 5315kg vs a launcher limit of 6530kg. This is a margin of 22.9% vs a required margin of 19.7%. Even adding in some desirable additional items and this only drops to 21.1% which is still healthy.

Power subsystem is fine. No news move along.

The desirable addional items include spreader rings for the solar shield to improve it's 3D structure, modifications to the shiled spreader bars and a trailing aft shield\trim tab that can be used for spacecraft balancing (at least I think that's what he said).

Mike then gave more depth on a specific problem relating to the cables that connect the ambient portion of the ISIM (at ~300K) and the cryosection (at ~35K). These are only 4m long and there is a 270K temperature drop across them that is not currently adequately cooled. They are adding a four stage cooling system to these to resolve this.

Finally he put up a slide showing the underside (sun facing side) of the spacecraft bus showing the potential location for a lightweight grapple fixture that could (in theory) be used to facilitate a repair mission at some stage. This is a ~2kg mounting so adding it isn't a major deal in itself. The audience gave it a mixed reaction and Mike certainly seemed to be presenting it as "someone else's idea, not mine".

Bob Giampaoli gave a brief update on some of the current work on the deployment mechanics that is ongoing. He specifically invited anyone who was interested to visit the JWST Solar-shield full scale mock-up at the HiBay at NG in Redondo Beach where they will be working on the detailed folding\unfolding mechanism until the end of the year.

They are not using pyro's at all for the unfolding\deployment. All mechanical using common locking equipment and deployment motors. I found this very interesting especially since there are a number of points where a single failure will be catastrophic.

The addition of the TMD's has also resolved issues for the deployment of the secondary mirror booms  makes it much easier to find an acceptable trade off between the stiffness required for the mirror mount and the flexibility required to deploy.

Overall the design is coming in within the Ariane's limits with some minor deviations that they expect to iron out pretty soon.

OTE and Wavefront Sensing Overview - Lee Feinberg

I was unable to capture the hard numbers properly on this and haven't had a chance to double check them so be especially careful of errors here. If it looks or smells wrong to you, then it probably is.

The WSC system is the tuning and aiming system that melds those 18 Beryllium hexagonal mirrors into one near perfect optical element. It leverages the NIRCAM as its imaging sensor. The use of adjustable optics here is to eliminate vibration and keep the mirror PSF stable to within 2%. The WSC will periodically (every 14 days) carry out a full re-work of it's magic to ensure that everything remains precise. Each of the 18 elements have an independent 6 degree of freedom mount using redundant actuators and a single radius of curvature control actuator. Mirror element assembly is at the point where they are just beginning to be polished, all the base machining and light weighting has been done.

Lee gave numbers on the progressive improvement in optical accuracy as the WSC process steps through its refining stages =>Basic Alignment => Focus adjustment => Coarse Phasing => Fine Phasing (to <1 Lambda) => Multi field Alignment (<0.5 Lambda ?). The final RMS deviation for a point source is < 0.35 micron. I think, I was struggling to keep up with this.

Finally and most importantly Lee walked through the various testing regimes, double checks and independent processes that will be used to prevent a repeat of the Hubble error. Main testing stages:

Tinsley (Polishing)  ambient testing Marshall  ambient and cryo testing Johnson SC  Full system testing (cryo again I think)An independent Test Plan Integrity and Review Team are in place to make absolutely sure there are no mistakes.The PDR for the OTE is in November.

One item that is interesting here is that Lee definitely said that the WSC process would be executed every 14 days but a number of documentations elsewhere say that wavefront control adjustments would only be needed at greater than 1 month intervals. I'm not sure when this changed. Overall it means that the mission will lose ~12 days observing time per year for this vs 6.

It seems to be one of the worst-kept secrets in Astrophysics, but my spies tell me that ESA and NASA will finally sign the MOU for JWST at Le Bourget on Monday. I gather there will also be an agreement on LISA Pathfinder.

BTW, if anyone gets an early copy of AvWeek I'm curious to know if they included a pic of the model in Dublin. My snail-mail copy may not arrive till JWST launches!

JWST Partner's Workshop - Dublin 11 June 2007 (Part II of III)NIRCam  Marcia Rieke University of Arizona (PI)The instrument is quite advanced now compared to some of the other systems due to the critical role it plays as both a science instrument and as the sensor for calibration and Wavefront Sensing and Control. The CDR took place in May 2006 and the instrument is now (well) into the Engineering Test Unit phase.

Selection of the HgCdTe Rockwell sensor units has started and Dr Rieke made a point of the fact that they had no problems getting good characteristics for the short wavelength units but the long wavelength sensors are proving a bit harder. This latter difficulty extends to NIRSpec also as this requires similar characteristics.

Overall NIRCam represents 2-3 orders of magnitude increase in sensitivity in the wavelength it is designed to cover  in particular the design goal was to hit sensitivity in the nanoJansky range and that is being achieved (see below).

As NIRCam is used as the sensor for the Wavefront Sensing and Control capability it has to be fully redundant. This has resulted in a design with 2 fully independent halves to the instrument covering a total FOV of 2.2'x4.4'. That 10 square arcminute FOV makes it well suited for wide area surveys in search of (rare) first light event. Its dual purpose also means that (according to Dr Rieke) it has been exquisitely designed in an optical sense.

Dr Rieke mentioned that NIRCam could\would be used for surface characterization of KBO's. This is obvious enough and part of the "Planetary Systems and Origins of Life" theme for the mission but this was one of many instances where the presenters were at pains to point out that JWST would be useful as an observatory for Solar System objects.

For those unfamiliar with the instrument the basic design is a dichroic refractive optic camera covering the 0.6 to 5 micron wavelength range allowing for two concurrent observations in short and long(er) wavelengths. The incoming beam is split into 0.6-2.3micron short band and 2.4  5.2 micron long band). There are coronographs available in both long and short modules.

Each short wavelength channel is directed to a 4096x4096 pixel sensor array comprised of a grid of 4 separate 2048x2048 pixel HgCdTe Rockwell sensors. The corresponding long wavelength channel is directed to a 2048x2048 pixel single HgCdTe Rockwell sensor. There are a total of 40 megapixels between the two halves.

* The sensitivity numbers (Jansky's) don't appear to be directly comparable to me. Anyone who can comment on the difference between sensitivity to achieve a "S/N of 5 over 5 orbits" (Hubble NICMOS) and "10 sigma over 10000 seconds" (NIRCam) please jump in. My understanding is that 10 sigma corresponds to an S/N of 100, if that is true then the above NIRCam numbers would need to be reduced by a factor of 20 in order to compare them with NICMOS. I believe that the 5 orbit number for Hubble equates to 5000 seconds but I'm not sure.

"A Pretty Picture is not Enough" or Imagery is Astronomy but Spectroscopy is Astrophysics.

NIRSpec is a multi object dispersive spectrograph that uses a MEMS shutter array to enable it to take up to 100 spectral samples concurrently. Sampling in one of three resolutions (R=100, R=1000, R=2700) using 2 x 2048x2048 HgCdTe Rockwell sensors. The twin detectors do not abut perfectly so there is a detector gap in the layout which mostly just causes targeting complications for R=100 sampling but causes dropout in the middle of spectra for many R=1000 samples and all R=2700 samples. These dropouts will require re-shooting the target using a different array location to recover the dropout regions.

In addition to its MOS mode it also supports an Integral Field Spectrograph mode and a classical long slit spectrograph mode.

Physically it's a monster 185kg mass measuring ~1.8mx1.4x1m 2m on a side. The prism\refraction wheel is sufficiently massive that it acts as an (undesirable) reaction wheel for the observatory.Internally it uses an all silicon carbide reflective optics (14 reflection) optical path.FOV is 3.4' x 3.6' with a 0.2milliarc second nominal slitwidth.

The MEMS shutters consist of 4 arrays of 365x171 micro shutters. In operation targets selection requires the opening of three shutters in a line perpendicular to the spectral spread direction  the central shutter covers the target and the shutters on either side are used for background removal. Combined with the fact that 2nd and 3rd order effects prevent the use of multiple collinear (in array terms) targets the effective maximum number of concurrent samples is ~100.

The MEMS arrays make this a very powerful instrument however manufacturing the array is extremely difficult and it is clearly pushing the limits of today's micro mechanical manufacturing expertise. The arrays are individually electrostatically latched open and reset in bulk magnetically (to closed). Manufacturing challenges mean that the arrays have clear salt and pepper effect flaws  Peter Jakobsen did not say what the error rate was but it was very obvious in the sample images he displayed. I would estimate that it was probably in the order of 2-3%. In general the fail closed flaws are more frequent than fail open flaws and happily fail-closed shutter flaws result only in aiming\planning problems as the only effect is that certain individual shutter locations cannot be used. Fail open flaws are much more problematic as any open shutter could contaminate a spectral sampling anywhere else on the same horizontal line of the array and so fail-open flaws result in the loss of an entire row of sampling locations from the MOS array (actually they reduce the effectiveness of the row immediately above and below also). Fortunately fail-open flaws can be converted to fail-closed flaws pre flight so at launch there should be no fail open flaws. Peter Jakobsen gave no indication of the expected reliability of the array. The detectors and the MEMS array are supplied by NASA.

In addition to the MEMS arrays there is an IFU image slicer with a a 0.1" resolution of 3"x3" and 5 fixed non interfering slits of width 0.1", 0.2" and 0.4".

More details here. http://www.eso.org/gen-fac/meetings/3Dspec...rribas_JWST.pdf Page 9 shows the MEMS shutter array, IFU and fixed slit layouts and the direction of dispersion relative to the detector layout. The detector gap is not shown but corresponds approximately to the mid point gap between the MEMS shutters.

Peter Jakobsen closed out with some additional comments about exoplanet spectroscopy. My notes say that he said the challenge there is that they will need to get the S/N ratio past 10^4, specifically capturing >10^10 photons into the detector in <1hr time frame. This difficulty arises (in part) because NIRSpec does not have a coronograph.

Group: Members
Posts: 76
Joined: 25-April 08
From: near New York City, NY
Member No.: 4103

A full-sized mockup of the James Webb Space Telescope is on display in lower Manhattan, New York City at Battery Park from now through June 6th. On Friday night, local astronomers will bring their telescopes to the site to show the planets and stars to people attending the panel discussion scheduled to include John Mather, John Grunsfeld, and Heidi Hammel, with journalist Miles OBrien moderating. Neil deGrasse Tyson, will host the stargazing party.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted.
Do not reproduce without permission. Read
here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the
individual posters and do not necessarily reflect the opinions
of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer
UnmannedSpaceflight.com moderation team is wholly independent
of The Planetary Society. The Planetary Society has no influence
over decisions made by the UnmannedSpaceflight.com moderators.

SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society
and is funded by donations from visitors and members. Help keep
this forum up and running by contributing
here.